1. Field of the Invention
The present invention relates to an optical demultiplexing method and an optical multiplexing method suitable for an optical transmission system which transmits a wavelength division multiplexed (WDM) light containing a plurality of signal lights of different wavelengths, and an optical transmission apparatus using the same, and in particular, relates to a technology for multiplexing/demultiplexing signal lights having different modulation rates.
2. Description of the Related Art
FIG. 13 is a schematic diagram showing one example of a configuration of a typical WDM optical transmission system. In this WDM optical transmission system, signal lights of different wavelengths output from a plurality of optical senders (OS) are wavelength division multiplexed by an optical multiplexer in a transmission terminal station 510, and thereafter, such a WDM light is amplified by an optical post-amplifier to be transmitted to an optical fiber transmission path L. Further, in an optical node apparatus 520 arranged on the optical fiber transmission path L, there are performed the optical add/drop multiplexing (OADM) of signal light of required wavelength on the WDM light being propagated through the optical fiber transmission path L, the wavelength dispersion compensation and/or the polarization mode dispersion compensation on the signal light waveform deteriorated due to the transmission, the compensation of power deviation among signal lights of respective wavelengths, and the like. Then, in a reception terminal station 530, the WDM light transmitted on the optical fiber transmission path L is amplified by an optical pre-amplifier, and thereafter, demultiplexed to each one wavelength by an optical demultiplexer to be received by optical receivers (OR) corresponding to the respective wavelengths. In such a WDM optical transmission system, the technique for multiplexing or demultiplexing the signal lights of respective wavelengths is required in the transmission terminal station 510, the optical node apparatus 520 and the reception terminal station 530.
For the WDM optical transmission system as described above, there increase demands for a system which wavelength division multiplexes signal lights having different modulating rates, to transmit a WDM light. As a conventional system satisfying these demands, there has been proposed a WDM optical communication system capable of efficiently arranging signal lights of a plurality of bit rates at different wavelength spacing (for example, 10 Gbit/s signal lights arranged at 50 GHz intervals, 40 Gbit/s signal lights arranged at 100 GHz intervals, and the like) (refer to Japanese Unexamined Patent Publication No. 2002-112294 and Japanese Unexamined Patent Publication No. 2001-345784).
In the case of the system which performs the WDM transmission of the signal lights having different modulating rates, as described above, since spectral bandwidths of the signal lights are different from each other depending on the modulating rates (bit rates) of the signal lights, any device needs to be made when the multiplexing/demultiplexing of the signal lights is performed. Namely, as one technology for multiplexing/demultiplexing the signal lights at high density arrangement intervals, such as 50 GHz and the like, there has been known a method in which the signal lights multiplexed/demultiplexed using, for example, an arrayed waveguide grating (AWG), a multi-layer film filter or the like, are further multiplexed/demultiplexed using an interleaver. When the signal lights are multiplexed/demultiplexed utilizing the interleaver, if the signal lights having different modulating rates are multiplexed/demultiplexed by the common interleaver, since the spectral bandwidths of the signal fights are different from each other according to the modulating rates, there is caused a problem in that signal qualities are deteriorated. Note, the interleaver is an optical multiplexer/demultiplexer having a function of demultiplexing a signal group at the certain wavelength spacing into even number channels and odd number channels to make a signal group having the wavelength spacing twice the above certain wavelength spacing, or contrary to the above, a function of multiplexing the even number channels and the odd number channels to make a signal group having the wavelength spacing half the above certain wavelength spacing.
To be specific, for example in a system which arranges 10 Gbit/s signal lights arranged at 50 GHz intervals to perform the WDM transmission, in the case of changing bit rates of part of the signal lights to 40 Gbit/s to upgrade the system (to increase the capacity of the system), if the WDM light is multiplexed/demultiplexed utilizing an interleaver of 50 GHz/100 GHz intervals corresponding to the 10 Gbit/s signal lights, since the spectral bandwidth of 40 Gbit/s signal light is wider than the spectral bandwidth of the 10 Gbit/s signal light, a signal component of 40 Gbit/s is leaked into an adjacent channel (crosstalk) and also the spectrum itself of the 40 Gbit/s signal light is subjected to the band restriction by the interleaver. On the other hand, in the case of using an interleaver of 100 GHz/200 GHz intervals corresponding to the 40 Gbit/s signal lights, although there is not caused a problem of crosstalk or band restriction, since the 10 Gbit/s signal lights are also transmitted at 100 GHz intervals, the frequency utilization efficiency is lowered so that an effect of upgrading cannot be achieved.
In order to avoid the above problems, in the invention disclosed in Japanese Unexamined Patent Publication No. 2001-345784, transmission characteristics corresponding to respective ports of the interleaver are devised so as to be asymmetrical with each other. Further, the present inventors have proposed a technology for combining to use a plurality of interleavers whose center frequencies in transmission bands are shifted to each other (refer to Japanese Patent Application No. 2003-083984) as shown in FIG. 14 for example.
Briefly explaining the summary of the prior invention referring to a configuration example of FIG. 14, in a WDM light given to an interleaver 540 of 100 GHz/200 GHz intervals, the 10 Gbit/s signal lights in 25 GHz bandwidth are arranged in the odd number channels and the 40 Gbit/s signal lights in 75 GHz bandwidth are arranged in the even number channels. This WDM light is demultiplexed by the interleaver 540, and thereafter, is further demultiplexed by interleavers 550 and 560 of 100 GHz/200 GHz intervals. Center frequencies in transmission bands of a port A1 of the interleaver 540 and a port B1 of the interleaver 550 are shifted by ±12.5 GHz from a center frequency of the 10 Gbit/s signal light, so that an overlapped portion of respective transmission bands of both ports is substantially coincident with a signal band of channel ch[4i-2] (i: integer of 1 or more). As a result, only the 40 Gbit/s signal light of the channel ch[4i-2] is taken out from the port B1. Further, overlapped portions of transmission bands of other ports are similarly set so as to be substantially coincident with signal bands of channels desired to be taken out.
However, in the multiplexing/demultiplexing technology corresponding to the signal lights having different modulating rates as described above, since the modulating rates are previously determined depending on wavelengths (frequencies) of the signal lights, there is a problem in that the modulating rates of the signal lights of respective wavelengths cannot be freely set.